Method and apparatus for the reproduction of images



Sept. 19, 1961 H. E. GUNNING ET AL 3,000,735 METHOD AND APPARATUS FOR THE REPRODUCTION OF IMAGES Filed June 11. 195e '2 z q: 3 /0 3/ j 17 l m jim /a mgl/ 115% 022 43 99 3a f2 xr *4 30 y W WW fix/g5.

layer, ,the photoemissive United States Patent O ilworth, Ill.

Filed June 11, 1956, Ser. No. 590,621

16 Claims. (Cl. 96-1) This invention relates generally to electrophotography and more particularly to methods and apparatus for producing images.

By exposing photoemissive or photoconductive layers or surfaces to radiation having a desired image or pattern, there is produced a corresponding emission of electrons in the areas receiving the radiation whereupon an electrostatic latent image is produced. The electric charge pattern which forms the electrostatic latent image may be rendered visible bly various developing procedures, for example, by depositing a suitable material such as finely divided'particles which come under the infiuence of the charge pattern.

In following the procedures outlined heretofore, it has been necessary to utilize bulky equipment oftentimes necessitating the use of high voltage generating equipment. Artisans in the field have been confronted with serious problems with respect to the uniformity of a charging field, the lack of flexibility with respect to the size of the images reproduced and the safety factors which are present in connection with the high voltage equipment employed.

In accordance with the principles of the present invention, it is contemplated to provide as a charging means in an electrophotographic process a layer formed from an electrically stressed substance and providing an effective working surface which exhibits a permanent electrostatic field as high as 30,000 volts per centimeter. A layered member so provided is sometimes referred to in the electrostatic arts as an electret.

In electrophotographically producing electrostatic irnages in accordance with the principles of the present invention, the photoemissive layer is brought into confronting proximate adjacency to one surface of the electret, whereupon radiation is projected onto the photoemissive layer in an image to be reproduced. The photoemissive layer may either be deposited directly upon the selected working surface of the electret, or to facilitate removability and portability of use, the photoemissive layer may be deposited on a backing sheet which is selectively brought into and out of proximate relation to the electret.

When the photoemissive layer is adjacent the electret, temporary illumination may be provided to uniformly charge the exposed surface of the photoemissive layer. Thereafter, the layer is grounded while the radiation in the form of an image to be reproduced is projected onto the photoemissive layer.

Alternatively, instead of illuminating the photoemissive layer may be temporarily grounded to discharge any stray electrostatic fields whereupon radiation is projected onto the photoemissive layer in an image to be reproduced.

In both sequences, an electrostatic image is produced which can be developed for reproduction purposes.

It is an object of the present invention, therefore, to provide electrophotographic methods and apparatus for producing electrostatic images and which overcome the deficiencies of the prior art.

Another object of the present invention is to provide a charging means for practicing electrophotographic meth- ICC ods and for use in electrophotographic equipment which is highly portable and compact.

Another object of the present invention is to provide a charging means for electrophotographic methods and for use in electrophotographic apparatus which eliminates the necessity of utilizing high voltage generating equipment.

Yet another object of the present invention is to provide a charging means for establishing uniform electrostatic fields, which charging means can be mass-produced with reproducibly uniform results.

Another object of the present invention is to provide a charging means for electrophotographic methods and apparatus affording greater potential gradients and capable of speedier operations through the use of greatly reduced exposure times.

A still further object of the present invention is to provide a charging means which is made up of a reduced number of simplified elements, convenient to handle and economical in cost.

Many other features, advantages and additional ohjects of the present invention will become manifest from the description which follows and the accompanying sheets of drawings in which preferred structural embodiments of the principles of the present invention are shown by way of illustrative example. It is believed that the methods disclosed and claimed will be most clearly understood from a description of the exemplary structural forms illustrated in the drawings.

On the drawings:

FIGURE l is a cross-sectional view and is somewhat diagrammatic in character of an electret provided in accordance with the principles of the present invention and coated on one working surface with a photoconductive layer;

FIGURE 2 is a view similar to FIGURE 1 but illustrating the use of an electret according to the principles of the present invention with a separable image-forming means constituting a backing sheet with a photoconductive layer deposited thereon;

FIGURES 4 and 5 are somewhat schematic front and end views, respectively, partly in section as indicated of a plate assembly according to the present invention showing an alternative means for charging the photoconductive layer of the electret device; and

FIGURE 3 is a somewhat schematic cross-sectional view of image-projecting means utilizing an electret charger in accordance with Ithe principles of the present invention.

\As shown on the drawings:

Although the principles of the present invention are of general utility, a particularly useful type of application of the principles of the present invention is made in connection with the electrophotographic methods and apparatus and specifically in connection with the production of electrostatic images for reproduction purposes.

As used herein, the term electret denotes a permanently electrified substance exhibiting electrical charges of opposite sign at its extremities.

Electrets are made from solid substances which are capable of being melted without chemical decomposition and which possess a finite dipole moment in liquid state, for example, carnauba wax, carnauba wax-beeswax mixtures, mixtures of carnauba wax with various synthetic, thermoplastic or polar resins such as polyvinyl chloride polymers, etc., soda-lime glass, Pyrex glasses, quartz, and sulfur.

A permanent charge is imposed upon the substance of the electret by allowing the liquid electret-forming substance to solidify in a strong electric eld. For example, the substance may be melted while interposed between two electrically-independent tinfoil electrodes at- 3 tached to opposite poles of a high voltage D.C. source furnishing a D.C. field of not less than 10,000 volts per centimeter. The liquid substance is allowed to remain in the Huid state within the field for at least one hour and is thereafter progressively cooled at a slow rate within the field until completely solidified.

Various forms of electrets, electret-forming materials, details of electret-making apparatus and various methods of preparing electrets found useful in the practice of the present invention are set forth in an article by Edward Padgett, entitled Improved Electrets in the April 1949 issue of Radio Electronics and in an article entitled The Electret by F. Gutmann, Physical-Chemical Laboratory, Faculty of Agriculture, University of Sydney, Sydney, Australia, appearing in volume 20, No. 3, of the July 1948 issue of Reviews of Modern Physics.

It has been found that electrets so formed are permanently volume-polarized and possess permanent electrostatic fields of opposite polarities at the extremities or surfaces thereof to which the electrodes were attached.

In the electrophotographic processes of the prior art, charges in the range of 4500 to 6000 volts per centimeter have been utilized. The permanent electrostatic fields possessed by electrets may range from a few hundred volts per centimeter and may exceed as high as 30,000 volts per centimeter at the working surfaces of the electret and, accordingly, it is contemplated according to the principles of the present invention that the electret will be provided with surfaces exhibiting a permanent electrostatic field in the order of magnitude of a few hundred volts to an upper limit determined by the breakdown potential of the air in contiguity to the charged surface.

Referring now to FIGURE l, an electret is indicated generally at and in this particular embodiment constitutes a flat plate-like member having a first surface 11 and a second surface 12 lying in spaced apart parallel planes. An exemplary spacing dimension between the surfaces l1 and 12 and corresponding to the thickness of the electret 10, would be one centimeter, although it will be appreciated that any suitable thickness could be utilized.

In the form of the invention illustrated in FIGURE l. the electret 10 is provided on the surface 11 with an electrically conductive layer 13 and the surface 12 is provided with an electrically conductive layer 14.

Circuit means are provided to electrically interconnect the conductive layers 13 and 14 and there is shown an electrical conductor 16 connected to the layer 13 and an electrical conductor 17 connected to the layer 14. 'Ihe circuit means are controlled by a switch 18 movable from a first position denoted by a contact member 19 Where the switch 18 is open, to a second position denoted by the contact member 20 connected to ground as at 21, and to a third position denoted by a contact member 22 forming the terminal end of the conductor 17. Thus, the switch 18 will isolate the conductive layers 13 and 14 at the first position, will ground the conductive layer 13 at the second position, and will electrically interconnect the conductive layers 13 and 14 at the third position.

As shown in FIGURE 1, a working surface 23 is presented by the conductive layer 13 on which is deposited a layer of photoconductive or photoemissive material 24. Any suitable photoconductive material may be employed but the material should be chosen to have an extremely low electrical conductivity in the dark. Sulfur is a suitable material for the purposes of the present invention although other materials are suitable and available for use as photoconductive layers, for example, naphtazarine, anaphthol, ,-naphthol, a-naphthylamine, quinizarin, an thracene, phenanthrene, stilbene, methylanthracene, chrysene, amorphous selenium and red phosphorus. In addition. there are many other aromatic or benzenoid compounds containing one or more benzene rings in a fused-ring structure which possess refractive indices exceeding two which form suitable photoconductors and which are available for use in connection with the practice of the principles of the present invention.

It may be noted that the material from which the conductive layers 13 and 14 are made can include any good electricallyA conductive type of material which does not react chemically with the photoconductor employed.

In the deposition of the photoconductive layer 24 on the metallic layer 13, care should be taken to assure intimate contact between the photoconductor and the metal. For example, with sulfur as the photoconductor and aluminum as the metal, the aluminum surface should first be roughened, either by abrasion or by chemical etching. The sulfur layer may be conveniently formed on the roughened surface of the aluminum by allowing a saturated solution of sulfur in carbon disulfide to evaporate on the metallic surface. After a uniform layer of sulfur has thus been deposited on the metal surface of a thickness not exceeding one millimeter, the sulfur is melted by heating the metal slightly above the melting point of the sulfur, whereupon the sulfur is maintained in liquid state for a few minutes to assure that any gas bubbles present at the interface of the metal with the molten surfur, or dissolved in either the metallic surface or in the sulfur be driven from the joint. Thereafter, the sulfur layer is cooled and solidifies. In order to make the photoconductive layer 24 of uniform thickness and of a depth sufficiently small to afford translucency, the cooled and solidified sulfur layer should be rubbed down with a fine abrasive.

In an electrophotographic process or in connection with a use with electrophotographic equipment, the electret 10, `as shown in FIGURE 1, may be mounted within an enclosure such as an image projection means indicated generally at 30 in FIGURE 3. The image projection means 30 comprises a camera-like apparatus including a support frame 31 carrying the electret 10 and having suitable means for accommodating the electrical conductors 16 and 17 associated with the circuit means of the electret 10. A bellows 32 connected to the support frame 31 provides a lightproof enclosure 33 and is connected at its opposite end to a lens support member 34 carrying an adjustable lens holder 36 supporting a pair of lenses 37 and 38. The front plate 34 also carries a lamp socket 39 in which is received a light bulb 40 forming a source of illumination within the enclosure 33. Suitable conductors 41 are electrically connected to the light source 40 and to an electrical source 42. To control the energization of the light source 40, a switch 43 is provided.

When the electret 10 is not in use, tion the switch 18 in a keeper position with the conductive layers 13 and 14 short circuited through the switch 18 at the contact member 22.

In practicing the electrophotographic process of the present invention, the plate assembly of the electret 10 is housed within the enclosure 33 and all visible light is excluded. It will be understood that the photoconductive layer 24 is positioned in registry with the lenses 37 and 38. With the plate assembly in the dark, the switch 18 is opened to the first position, thereby removing the short circuit of the pole faces of the electret 10, whereupon the charge field potential of the electret 10 will be asserted. The face or surface 11 of the electret 10 being adjacent the conductive layer 13 will induce an electrostatic charge on the working surface 23, which charge will be exhibited as an electrostatic field uniform over the entire surface 2 3 and having a magnitude in the order of at least about 4500 volts per centimeter.

The photoconductive layer 24 is then subjected to a uniform illumination over its entire surface. In a camera this is easily accomplished by focusing the lens or lenses of the camera on a light colored object such as a white screen or other uniformly reflecting or illuminated surit is desirable to posiface. For clarity and understanding, however, it will be understood that the light source 40 may be energized to produce illumination over the entire photoconductive surface 24.

Under the uniform illumination, the photoconductive layer 24 becomes an electrical conductor and the electret induces a uniform electrostatic charge on the surface of the photoconductive layer.

The switch 18 is then moved to the second position engaging the contact 20, the plate assembly of the electret 10 once again being in total darkness within the enclosure 33 and all visible light being excluded from the enclosure 33. Thus, the conductive layer 13 is grounded and the charge on the photoconductive layer 24 is isolated since the current flux from the photoconductive layer 24 is negligible in darkness.

To control the projection of radiation in the form of an aerial image, the image-projecting means or camera 30 is provided with a shutter, shown schematically at 44. The camera shutter 44 is opened and the aerial light image is focused on the charged photoconductive surface of the layer 24, while the switch 18 is in contact with the contact 20, thereby grounding the conductive layer 13. In other words, since the photoconductive layer consists of material which emits electrons when exposed to radiation or which becomes conductive as a function of the illumination thereon, the projection of radiation in accordance with a desired image or pattern produces in the photoconductive layer and at the areas receiving the radiation, an electric charge pattern which forms an electrostatic latent image. Thus a positive electrostatic latent charge image is formed on the surface of the photoconductive layer 24, through the transfer of charge via the conductive layer 13 to ground. The rate of latent image formation will be determined in general by the charge on the electret 10, the thickness and composition of the photoconductive layer 24 and the intensity and spectral distribution of the light employed in forming the aerial light image.

As an alternative method of exposing the photoconductive layer 24, the photoconductive layer 24 may be uniformly charged as described above by exposing to uniform illumination whereupon the electret 10 is turned over with the pole face 11 formerly in contact with the conductive layer 13 in contact with the conductive layer 14 and the pole face 12 of the electret 10 in contact with the conductive layer 13. To accomplish such reversal, it will be understood that the electret assembly must be so constructed as to facilitate disassembly and reversal of position of the component parts. By the reversal described, the electret 10 induces a charge on the conductive layer 13 of opposite polarity to the charge resident upon the surface of the photoconductive layer 24. Consequently, the potential gradient of the electrostatic eld will be higher and will facilitate formation of an electrostatic latent image upon projection of radiation in the form of an aerial image onto the photoconductive layer 24. A positive image will be formed with the lightest areas of the latent image showing the lowest surface electrostatic charge and the darkest areas showing the highest surface charge. The higher potential gradient established by reversing the pole faces of the electret 10 has the effect of speeding the process since exposure time may be cut down.

It will be understood that the higher electrostatic potential gradient provided by reversing the position of the electret assembly components may also be accomplished by moving the photoconductive layer 24 and its backin-g to a second Velectret similar to the electret 10 but against the surface thereof having a charge of opposite polarity.

In FIGURE 2, an alternative form of construction of the electret is shown, similar reference numerals being employed but bearing the suix a to indicate similar structural features in connection with the electret 10. The electret layer is shown at 10a and provides faces 11a and 12a, each having its conductive layer 13 and 14, respectively.

In this form of the invention, the image-forming member is separated from the electret and comprises a backing sheet made of conductive material such as aluminum or the like and shown in FIGURE 2 at 50. A photoconductive layer 51 coats a surface 52 formed on the backing layer 50 and the opposite surface 53 is adapted to be placed against the working surface 23 of the conductive layer 13. The circuit connections are similar to those already described in connection with the electret of FIGURE 1 and`like reference numerals have been applied. It will be understood that the removability of the image-forming member affords great exibility in the practice ofthe methods herein described and is desirable in facilitating the formation of the electrostatic image at one location, for example, in a camera and developing the latent electrostatic image at another location such as a dusting chamber.

In FIGURES 4 and 5, analternative method and means for charging the photoconductive layer is illustrated. An electret 10b has an image-forming member 60 placed thereon characterized by the formation on the surface of a photoconductive layer 61. At the sides of the electret 10b are track-forming means 62 movably supporting a charging bar 63.

With the photoconductive surface 61 discharged and then put in total darkness, a switch 64 is positioned to interconnect the electret conductive layer, shown at 66, through a contact indicated at 67, through a conductor 68 and to the charging bar 63. The charging bar 63 is then moved back and forth across the surface of the photoconductive layer 61, preferably about one millimeter above the surface. In this connection, those versed in the art will appreciate that the distance between the photoconductive layer 61 and the charging bar 63 will depend upon the surface potential gradient of the electret 10b. It will be further noted that the charging bar 63 extends across the entire width of the electret 10b for the purpose of covering the corresponding area prescribed by the photoconductive layer 61. Accordingly, the charging bar 63 will move over the entire working surface of the photoconductive layer 61. The bar 63 is then pulled to one side and it will be understood that all stray electrostatic fields will have been removed from the photoconductive layer.

With the bar 63 pulled aside and the system in darkness, the switch 64 is moved into contact with the contact member 69, thereby grounding the electret conductive layer indicated at 70. The photoconductive layer 61 is then ready to receive the aerial image and it will be noted that a positive electrostatic image will be formed.

In so charging the plate, the mechanical work associated with the motion of the charging bar will be imparted to the electrons moving towards the surface of the photoconductive layer 61, thereby increasing the efficiency of the charging procedure.

It is' to be understood that during the process of exposure of the uniformly charged photoconductive surface or layer 24, 51 or 61 to an aerial light image, the rate of dissipation of the surface charge will be a direct function of the localized light intensity impinging upon any given element of area of the photoconductive surface or layer. Upon prolonged exposure of the charged photoconductive surface to the aerial light image, the areas of highest illumination corresponding to the most highly light-reiiective areas of the image to be reproduced will ultimately be completely discharged. Furthermore, since all surfaces reilect some light, there will normally be no region of complete darkness in the aerial light image. We have found that maximum contrast in the developed plate can be achieved when the exposure is limited to that period of time wherein the most highly illuminated area of the photoconductive surface will still retain an appreciable surface charge.

Having produced the latent electrostatic image, it will be understood that any of the well known developing procedures may be utilized to render the latent image visible. For example, the image member may be dusted with finely divided particles which, under infiuence of the charge pattern, will render the latent image visible.

In this connection, it may be noted that desirable results may be obtained by precharging the finely divided particles so that the polarity of the charge on the particles will bear a definite relation to the* charge pattern in the photoconductive layer.

In charging the plate, as described in connection with FIGURES 4 and 5, and then subsequently projecting radiation onto the photoconductive layer 61, in the form of an image, the highest surface charge will occur at the portions of the charge pattern corresponding to the lightest part of the image. In other words, the image is negative rather than positive. In accordance with the principles of the present invention, a dusting powder of finely divided particles can be utilized which is charged with opposite polarity, whereby the developed electrostatic image will be a positive image.

Although the exemplary form of the electret herein described is a fiat plate-like element, it will be understood that the electret can be formed in any configuration affording two independent surfaces operable as pole faces, for example, a cylinder, a hollow hemisphere or other suitable shapes.

Although minor modifications might be suggested by those versed in the art, it should be understood that we wish to embody within the scope of the patent warranted hercon all such modifications as reasonably and properly come within the scope of our contribution to the art.

We claim as our invention:

l. The method of electrophotographically producing an electrostatic image which includes the steps of (l) bringing into confronting proximate adjacency two layers of substantially coextensive area including a first photoconductive insulating layer and a second layer formed from an electrostatically volume-polarized electret material possessing permanent electrostatic fields of opposite polarities at opposed surfaces thereof, (2) inducing an electrostatic charge on the photoconductive layer with said electret material and (3) while said photoconductive insulating layer is grounded, projecting radiation on said photoconductive insulating layer in an image t be reproduced by exposure to light, said steps being performed sequentially in the order named.

2. The method of claim l in which the photoconductive insulating layer is temporarily illuminated to uniformly charge the exposed surface thereof while said electret is grounded, said further step being performed after step (l).

3. The method of claim 1 which includes the further step of developing the latent electrostatic image to render the said image visible.

4. The method of claim 3 in which the charge is induced by temporarily illuminating the photoconductive insulating layer to uniformly charge the exposed surface thereof and thereafter grounding the photoconductive insulating layer, said further steps being sequentially performed between said steps (l) and (3) and wherein said step of developing the latent electrostatic image consists of dusting said first layer with a finely divided material forming a plurality of particles subject to the influence of the charging pattern.

5. The method of claim 3 in which prior to step (2) there is included the further step of temporarily grounding the first layer to discharge any stray electrostatic fields on said first layer, and wherein the step of developing said latent electrostatic image consists of dusting the first layer with finely divided particles charged with a polarity opposite to that of the surface of the layer.

6. The method of claim l plus the additional step of moving a conductive member electrically connected to one pole face of the electret through a plane in proximity to the photoconductive insulating surface, thereby to insure a uniform charge condition on the photoconductive insulating surface.

7. The electrophotographic method of reproduction which includes the steps of coating a photoconductive insulating layer on a surface of a volume-polarized electret material possessing an electrostatic field at said surface, said layer including an electrostatic charge thereon, and projecting radiation onto the layer by exposure thereof to light in an image to be reproduced to form a latent electrostatic image, and developing the image to render it visible.

8. In combination, a charging means comprising a first layer of volume-polarized electret material possessing permanent electrostatic fields of opposite polarities at opposed first and second spaced parallel surfaces thereof, a second layer of electrically conductive material on each of said first and second surfaces, each of said second layers being substantially coextensive in area with a corresponding one of said surfaces, and image-forming means having a photoconductive insulating layer adjacent one of said second layers, whereby radiation may be projected onto said photoconductive insulating layer by exposure to light in an image to be reproduced.

9. In combination, a charging means comprising a first layer of volume-polarized electret material possessing permanent electrostatic fields of opposite polarities at opposed first and second spaced parallel surfaces thereof, a second layer of electrically conductive material on each yof said first and second surfaces, each of said second `layers being substantially coextensive in area with the corresponding one of said surfaces, and image-forming means having a photoconductive insulating layer adjacent one of said second layers, whereby radiation may be projected by exposure to light onto said photoconductive insulating layer in an image to be reproduced, and circuit means electrically interconnecting said second layers and including a switch movable to a closed position for preserving the charge in said first layer and being movable into an open position whereby the electrostatic fields are asserted at said first and second surfaces.

l0. In combination, a charging means comprising a first layer of a Volume-polarized electret material possessing permanent electrostatic fields of opposite polarities at opposed first and second spaced parallel surfaces thereof,

" a second layer of electrically conductive material on each of said first and second surfaces, each of said second layers being substantially coextensive in area with the corresponding one of said surfaces, and image-forming means having a photoconductive insulating layer adjacent one of said second layers, whereby radiation may be projected onto said photoconductive insulating layer in an image to be reproduced by exposure to light, said imageforming means comprising a photoconductive insulating layer deposited in a thin film on one of said first and second surfaces.

1l. In combination, a charging means comprising a first layer of a volume-polarized electret material possessing permanent electrostatic fields of opposite polarities at opposed first and second spaced parallel surfaces thereof, a second layer of electrically conductive material on each of said first and second surfaces, each of said second layers being substantially coextensive in area with the corresponding one of said surfaces, and image-forming means having a photoconductive insulating layer adjacent one of said second layers, whereby radiation may be projected onto said photoconductive insulating layer in an image to be reproduced by exposure to light, said image-forming means comprising a backing sheet of electrically conductive material and having a photoconductive layer deposited in a thin lm over said backing sheet.

l2. In a camera, a volume-polarized electret member having a photoconductive insulating surface on one pole face and a conductive layer on an opposite pole face, a conductive scanning member electrically connected to said conductive layer, and track means confining said scanning nember for a scanning movement through a plane in )roximity to said photoconductile insulating surface, ,hereby to make the charge condition on said photoconuctive insulating surface uniform.

13. The use of an electret member in electrophotography which includes the steps of placing a volume-polarized electret member having a permanently charged surface exhibiting an electrostatic eld in the order of from at least about 200 volts per centimeter to the breakdown potential of an adjacent air layer in contiguous relation to a photoconductive insulating layer, inducing an electrostatic charge on the photoconductive insulating layer with said electret member, and exposing said photoconductive insulating layer to an image to be photographed by exposure to light, thereby producing an electrostatic latent image on the surface of said photoconductive insulating layer.

14. The use in electrostatic image production of an electret member which includes the steps of forming permanently precharged surfaces exhibiting a permanent electrostatic iield in the order of magnitude of at least about 200 volts per centimeter by volume-polarizing an electret material, bringing one of said surfaces of said volume-polarized electret material adjacent an imageforming member having a photoconductive insulating surface formed thereon, and inducing an electrostatic charge on said photoconductive insulating surface with said volume-polarized electret material.

15. The method of producing an electrostatic image which includes the steps of forming permanently charged surfaces atly shaped to have spaced parallel surfaces by volume-polarizing an electret material, contacting a layer of photoconductive insulating material having electrical conductivity characteristics varying from a minimum in darkness to a maximum when exposed to light against one of said permanently charged surfaces, temporarily exposing said layer to a uniform source of illumination and concurrently therewith inducing a uniform charge on the surface of said layer, and temporarily grounding said layer while exposing said photoconductive insulating layer to an image to be reproduced by exposure to light, thereby to form an electrostatic image in the layer.

16. In an electrophotographic device of the type adapted to focus an image on a photoconductive insulating layer for reproducing said image, the improvement of charging means for establishing an electric Iield comprising a volume-polarized electret material, and means retaining said electret material adjacent the photoconductive insulating layer to exhibit a permanent electrostatic eld at said layer.

References Cited in the file of this patent UNITED STATES PATENTS Paggett, Radio Electronics, April 1949, pp. 20-23. Review of Modern Physics, vol. 20, No. 3, July 1948, p. 470. 

1. THE METHOD OF ELECTROPHOTOGRAPHICALLY PRODUCING AN ELECTROSTATIC IMAGE WHICH INCLUDES THE STEPS OF (1) BRINGING INTO CONFRONTING PROXIMATE ADJACENCY TWO LAYERS OF SUBSTANTIALLY COEXTENSIVE AREA INCLUDING A FIRST PHOTOCONDUCTIVE INSULATING LAYER AND A SECOND LAYER FORMED FROM AN ELECTROSTATICALLY VOLUME-POLARIZED ELECTRET MATERIAL POSSESSING PERMANENT ELECTROSTIC FIELDS OF OPPOSITE POLARITIES AT OPPOSED SURFACES THEREOF, (2) INDUCING AN ELECTROSTATIC CHARGE ON THE PHOTOCONDUCTIVE LAYER WITH SAID ELECTRET MATERIAL AND (3) WHILE SAID PHOTOCONDUCTIVE INSULATING LAYER IS GROUNDED, PROJECTING RADIATION ON SAID PHOTOCONDUCTIVE INSULATING LAYER IN AN IMAGE TO BE REPRODUCED BY EXPOSURE TO LIGHT, SAID STEPS BEING PERFORMED SEQUENTIALLY IN THE ORDER NAMED. 